This invention relates to a method for producing synthetic hydrotalcite.
Hydrotalcite is a naturally occurring mineral having the formula 6 MgO.Al.sub.2 O.sub.3.CO.sub.2.12 H.sub.2 O or Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4 H.sub.2 O. Known deposits of natural hydrotalcite are very limited and total only about 2,000 or 3,000 tons in the whole world. Natural hydrotalcite has been found in Snarum, Norway and in the Ural Mountains. Typical occurrences are in the form of serpentines, in talc schists, and as an alteration product of spinel where, in some cases, hydrotalcite has formed as pseudomorphs after spinel.
The upper stability temperature of hydrotalcite is lower than the lower limit for spinel. Spinel and hydrotalcite theoretically never would appear together in stable condition. If equilibrium has been established, the spinel would be completely changed to hydrotalcite. However, naturally occurring hydrotalcite is intermeshed with spinel and other materials.
Natural hydrotalcite is not present as pure product and always contains other minerals such as penninite and muscovite and potentially undesirable minerals such as heavy metals. Conventional practice recognizes that it is practically impossible to remove such impurities from a natural hydrotalcite.
Previous attempts to synthesize hydrotalcite have included adding dry ice or ammonium carbonate (a) to a mixture of magnesium oxide and alpha-alumina or (b) to a thermal decomposition product from a mixture of magnesium nitrate and aluminum nitrate and thereafter maintaining the system at temperatures below 325.degree. C. at elevated pressures of 2,000-20,000 psi. Such a process is not practical for industrial scale production of synthetic hydrotalcite by reason of the high pressures. Furthermore, the high pressure process forms substances other than hydrotalcite, such as brucite, boehmite, diaspore, and hydromagnesite.
Ross and Kodama have reported a synthetic mineral prepared by titrating a mixed solution of MgCl.sub.2 and AlCl.sub.3 with NaOH in a CO.sub.2 free system and then dialyzing the suspension for 30 days at 60.degree. C. to form a hydrated Mg-Al carbonate hydroxide. The mineral product has been associated with the formula Mg.sub.6 Al.sub.2 CO.sub.3 (OH).sub.16.4 H.sub.2 O while having the properties of manasseite and hydrotalcite. X-ray diffraction powder patterns have indicated that the mineral more closely resembles manasseite than hydrotalcite, while the differential thermal analysis curve of the precipitate has been characterized as similar to that given for hydrotalcite.
Kerchle, U.S. Pat. No. 4,458,026, discloses a preparation of Mg/Al/carbonate hydrotalcite which involves the addition of mixed magnesium/aluminum nitrates, sulphates or chlorides as an aqueous solution to a solution of a stoichiometric amount of sodium hydroxide and carbonate at about 25.degree.-35.degree. C. with stirring over a several-hour period producing a slurry. The slurry is then heated for about 18 hours at about 50.degree.-200.degree. C. (preferably 60.degree.-75.degree. C.) to allow a limited amount of crystallization to take place. After filtering the solids, and washing and drying, the dry solids are recovered.
Kumura et al. U.S. Pat. No. 3,650,704, reports a synthetic hydrotalcite preparation by adding an aqueous solution of aluminum sulfate and sodium carbonate to a suspension of magnesium hydroxide. The suspension then can be washed with water until the presence of sulfate radical becomes no longer observable. The suspension is heated at 85.degree. C. for three hours and dried. The magnesium component starting material is reported as any member of the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate, and water-soluble magnesium salts, e.g., such as mineral acid salts including magnesium chloride, magnesium nitrate, magnesium sulfate, magnesium dicarbonate, and bittern.
It is an object of the present invention to produce synthetic hydrotalcite in high purity.
It is another object of this invention to produce hydrotalcite in high yield at atmospheric pressure.